Metal halide lamps emit radiation at wavelengths above 200 nm. That portion of the emitted radiation falling between 150-400 nm is ultraviolet (UV) radiation, which is harmful to human eyes and skin and which also causes fading, discoloration and degradation of fabrics, plastics and paints. In addition to the harmful results of UV radiation which escapes a lamp, the UV radiation also causes problems within the lamp itself. A need exists, therefore, for a means for blocking, and preferably for eliminating, the emission of UV radiation.
Conventional arc tubes may employ a glass outer jacket to prevent the external emission of UV radiation. Traditional metal halide lamps did not encounter the problems of today's progressive products because the arc tube was generally pure quartz enclosed in a glass outer jacket to provide UV protection. Newer products, however, such as fiber optic sources and automotive lamps, encounter size and use constraints which prohibit the use of outer jackets. Doped quartz has therefore been used in thin-walled arc tubes to absorb the UV emissions from the arc within the arc tube itself, thus eliminating the need for a glass outer jacket.
The use of doped quartz, as set forth in U.S. Pat. No. 5,196,759, is suitable for conventional thin-walled lamps. It is not suitable, however, for use with thick-walled arc tubes, where the combination of hot spot devitificaton with doped quartz, causes enhanced devitrification and shortened lamp life. In thick-walled arc tubes, the UV radiation from the arc causes thermal non-uniformity across the arc tube wall. Naturally occurring "hot spots" within the arc tube wall, which are located directly above the discharge in metal halide arc tubes, are due to natural convection within the tube. These hot spots weaken the arc tube wall causing accelerated devitrification and eventual failure of the lamp. This problem, inherent in all metal halide lamps, is compounded when doped quartz is used in thick-walled arc tubes, because emitted UV radiation is absorbed preferentially along the inside wall of the arc tube due to the doping. This heat energy, in combination with the already present hot spot, causes accelerated devitrification and lamp failure.
A further option for UV protection is the use of coatings on the outer surface of the arc tube. One such option is set forth in U.S. Pat. No. 5,336,969, which described the use of a coating comprising a suspension of CeF.sub.3 and Al.sub.2 O.sub.3 .cndot.SiO.sub.2 which is applied to the arc tube surface and fired. This coating operates to absorb UV transmission. Another option is set forth in U.S. Pat. No. 4,949,005 which discloses a tantala-silica interference filter used on a tungsten halogen lamp to reflect or absorb specific wavelengths by variation in filter design.
While various types of coatings which merely absorb UV radiation are known in the lamp industry, these coatings do not address the inherent devitrification problems present in thick-walled metal halide arc tubes. Therefore, a need exists for a coating for application to thick-walled arc tubes which will absorb UV radiation to prevent external emission and eliminate accelerated devitrification due to the thermal energy deposited on the arc tube wall.
To this end, the subject multi-layer coating composition, suitable for use on thick-walled metal halide lamps, solves all of the foregoing problems. It eliminates external UV radiation. Internally, that is within the lamp envelope, the subject coating functions to enhance vaporization of the metal halide components of the arc tube and decrease devitrification, i.e. lamp life is prolonged. The coating also functions to relieve the arc tube of the compounded effects of hot spots in combination with UV energy, which affect the mechanical strength of the arc tube.